Sora no Seibun
The main characters are Okamoto and Koizumi. Okamoto is an athlete, more preciously a basketball player. He just entered his high school just to be with his girlfriend, Yumi. There is a backstory between Okamoto and Yumi. Yumi has a large ugly scar on her back. Okamoto accidently dropped a boiling kettle on her back when they were elementary kids. Since then, Yumi threatened Okamoto to be responsible for what he had done by forcing Okamoto to be Yumi's man. Even though it does not clearly show, they regularly have sexual relationships. All that changes when the short and skinny Koizumi joins the basketball club. Koizumi is generally described as cute and feminine. You cannot even tell whether he is male or female in casual clothing.